Biodegradable polymers, such as those belonging to the family of polylactic acid (PLA) and polyglycolic acid (PGA), are widely used for fabricating implantable devices. Such devices are currently used for drug delivery, joint resurfacing (using allograft chondrocytes and synthetic polymer scaffolds), and fracture fixation in medicine, particularly in the fields of orthopaedics, podiatry and maxillofacial surgery. The degradation of these materials has been studied both in vivo and in vitro. It has been reported that they degrade primarily by hydrolysis of ester bonds. Upon degradation, these materials release acidic by-products, which then enter the tricarboxylic acid cycle and are reduced to carbon dioxide and water.
Basic substances, such as calcium carbonate and hydroxyapatite, have been described to some extent in the literature in a variety of applications. For example, Kampner.sup.7 describes a permanent joint prosthesis having a biodegradable anchor of glycolic acid and polylactic acid polyesters with calcium carbonate and hydroxyapatite. The Fong et al. patent.sup.6 employs sodium hydroxide in microspheres, than sodium hydroxide providing for the regulation of microsphere core material release. The Kampner patent.sup.7 refers to hydroxyapatite in polymeric bone implants, and Allmann et al..sup.8 refers to enhancing drug loading efficiency of PLA nanoparticles by using a savoxepine base and modifying the pH of an aqueous base. Basic substances thus have been used to increase drug loading efficiency or to increase the solubility of an active ingredient.
Several studies have reported on the effects of pH change during biodegradable polymer breakdown. For example, Younes et al..sup.9 reports a relatively greater polymer mass loss with increasing pH.
Other investigators in the area of biodegradable implantable materials have raised questions about the biocompatibility and toxicity of biodegradable polymer breakdown products.sup.1-3. For example, Bostman et al..sup.1 reported aseptic sinus formation with biodegradable implants used to repair fractures in humans. Lowered pH in the vicinity of an implantable device from breakdown of PLA and PGA breakdown has also been suggested to cause adverse effects like inflammation and tissue damage.sup.2,3. However, changes in pH that occur in vivo with polymer implant degradation have not been reported to significantly affect physiological levels of blood components. For example, Vasenius et al..sup.11 report "normal" results for blood components and acid base balance in vivo with implanted rods of the biodegradable polymers polylactic acid or poly D, L lactic acid.
Solving the problem of controlling pH shifts due to polymer breakdown products would improve the biocompatibility of a variety of implantable devices for both short term and long term use in the medical industry. A method for controlling pH would also be useful in other industries where pH changes from polymer degradation present a problem.